The present invention relates to an image forming apparatus such as a copying machine, a printer, etc., which employs an electrophotographic or electrostatic recording method. It also relates to a developing apparatus suitable for an image forming apparatus employing such a developing apparatus.
In the past, as an electrophotographic employing an electrophotographic or electrostatic recording method, an apparatus having the structure shown in FIG. 18 has been known. FIG. 18 is a schematic drawing of an example of image forming apparatus in accordance with the prior art, for showing the basic structure thereof.
In the case of this image forming apparatus, a photoconductive drum 100 is employed as an image bearing member. The photoconductive drum 100 comprises: a cylindrical substrate formed of, for example, aluminum; and a photoconductive layer (organic photoconductive layer, for example) coated on the peripheral surface of the cylindrical substrate. It is rotationally driven. Disposed around the photoconductive drum 100 are a charge roller 101, an unshown laser beam type scanning optical system, a developing device 102 as a developing means, a transfer roller 111, and a cleaner 109, which are listed in order, in terms of the rotational direction of the photoconductive drum 100.
After the peripheral surface of the photoconductive drum 100 is uniformly charged by the charge roller 101, it is exposed to the beam of laser light projected in the scanning manner from the unshown optical system. As a result, an electrostatic latent image is formed on the peripheral surface of the photoconductive drum 100. This electrostatic latent image is visualized (developed) with the use of the toner (developer) in the developing device.
Recording medium, which in this case is a piece of transfer medium P, is fed into the main assembly of an image forming apparatus, through an unshown sheet feeding opening, and is delivered, in synchronism with the above described formation of the visible image (toner image), to the area in which the peripheral surfaces of the photoconductive drum 100 and transfer roller 111 are virtually in contact with each other, and in which the visible image (toner image) is transferred onto the transfer medium P. Thereafter, the image on the transfer medium P is welded to the transfer medium P by a fixing device 110.
The developing device 102 comprises: a development roller 108 as a developer bearing member; a supply roller 105 for supplying nonmagnetic single-component toner (negative in inherent polarity) to the development roller 103; a stirring member 106 for conveying the toner within the container to the adjacencies of the supply roller 105; a development blade 104 as a developer amount regulating member for regulating the amount of the toner on the peripheral surface of the development roller 103; etc.
Since the development roller 103 is placed in contact with the photoconductive drum 100, it is formed of elastic substance. The development blade 104 is placed in contact with the peripheral surface of the development roller 103, with the use of the resiliency of a piece of thin springy metallic plate, generating a small amount of contact pressure against the peripheral surface of the development roller 103.
In order to transfer the toner from the development roller 103 onto the photoconductive drum 100, development bias is applied to the development roller 103 from a development bias power source 107 so that the development roller 103 is charged to a predetermined potential level. Further, in order to stabilize the electric charge of the toner, a blade bias power source 108 is connected to the development blade 104, and blade bias is applied to the development blade 104 so that the development blade 104 is charged to a predetermined potential level (Japanese Laid-open Patent Application 05-011599). There are various blade bias power sources (108); for example, those which are identical in potential as the development bias 107, those which are different in potential from the development bias 107, etc.
As described above, in the case of an image forming apparatus in accordance with the prior art (which hereinafter may be referred to as a conventional image forming apparatus), fixed DC bias or AC bias is applied to the development blade 104 in an attempt to stabilize the toner in terms of electric charge, manner in which the toner is coated, etc.
However, in the case of a conventional image forming apparatus, it was difficult to stabilize the toner coat while preventing the toner from scattering, preventing the reversal toner from solidifying on the development blade, and also, preventing the toner from welding itself to the development blade.
For example, when an image forming operation was carried out using one of the conventional image forming apparatuses, the following problems could be observed.
Firstly, when the potential of the development bias power source 107 was rendered the same as that of the blade bias power source 108, the image forming apparatus began to form images irregular in density, more specifically, images having unwanted vertical streaks across the halftone areas thereof, after the formation of approximately 1,000 copies. The streaks similar to the vertical streaks of the images were found also on the portion of the peripheral surface of the development roller 103, on the immediately downstream side with respect to the development blade 104, in terms of the rotational direction of the development roller 103.
The visual examination of the development blade 104 revealed that agglomerations of toner had welded to the area a (area next to downstream border of contact area between development blade 104 and development roller 103, in terms of rotational direction of development roller 103) of the development blade 104. Obviously, the toner particles in the toner layer, the positions of which corresponded to those of the agglomerations of the toner welded to the development blade 104, were blocked by the agglomerations. As a result, the toner layer on the peripheral surface of the development roller 103 became thinner, across the areas corresponding in position to the areas of the development blade 104 having the toner agglomerations, effecting therefore images suffering from density anomaly in the form of unwanted vertical streaks.
The cause of the above described problem will be described with reference to FIG. 19 which is a schematic drawing for showing the process in which the toner particles weld in agglomeration to the development blade. Designated by a referential code T are negatively charged nonmagnetic single-component toner particles. The developer is a mixture of the toner particles T and external additive particles G1 as auxiliary particles.
The toner particles borne on the peripheral surface of the development roller 103 are negatively charged particles. In the normal ambience, after passing the development blade 104, the potential of the toner layer on the peripheral surface of the development roller 103 is in the range of approximately xe2x88x9220-xe2x88x9250 V (measured with Surface Potentiometer Model 1334: Treck Co., Ltd.). In the contact area between the development roller 103 and development blade 104, the surface of the development blade 104 is constantly rubbed by the toner particles. Therefore, the external additive particles are not likely to remain adhered to the surface of the development blade 104.
However, on the downstream side of the contact area between the development roller 103 and development blade 104 (area a in drawing), the distance between the toner layer and the surface of the development blade 104 gradually increases, creating potential gradient.
In other words, even though the development roller 103 and development blade 104 remain the same in potential, the surface portion of the toner layer on the peripheral surface of the development roller 103 becomes greater in negative potential than the development blade 104, because of the potential of the negatively charged toner particles in the toner layer on the development roller 103.
Thus, the negative charged external additive particles G1 in the surface portion of the toner layer move away from the layer, and adhere to the area a of the surface of the development blade 104. The amount of these external additive particles G1 adhering to the area a of the surface of the development blade 104 gradually increases with the increase in the cumulative usage of the image forming apparatus, substantially increasing the apparent roughness of the surface of the development blade 104.
Then, the roughened surface of the development blade 104 is rubbed by the toner layer. As a result, some of the toner particles in the toner layer are welded in agglomeration to the surface of the development blade 104 by the frictional heat. This is the theory behind the welding, in agglomeration, of toner particles to the surface of the development blade 104.
In the case of the conventional image forming apparatus disclosed in Japanese Laid-open Patent Application 05-011599, a voltage of xe2x88x92300 V is constantly supplied to the development roller 103 from the development bias power source 107, and a voltage of xe2x88x92400 V is supplied to the development blade 104 from the blade bias power source 108, creating a potential difference of approximately 100 V between the development roller 103 and development blade 104. Therefore, the negatively charged external additive particles G1 do not move away from the surface of the toner, in spite of the electric charge which the toner layer has.
However, when a potential difference was provided between the development roller 103 and development blade 104 as disclosed in Japanese Laid-open Patent Application 05-011599, the follow problems occurred.
For example, after the formation of approximately 1,500 copies, the image forming apparatus began to form copies which were low in density across the solid areas and/or suffered from unwanted vertical streaks. This problem was found also on the portion of the peripheral surface of the development roller 103, on the immediately downstream side of the contact area between the development roller 103 and development blade 104, in terms of the rotational direction of the development roller 103.
The visual examination of the development blade 104 carried out to find the causes of this problem revealed that the toner particles had adhered to the entirety of the area b (area next to upstream border of contact area between development blade 104 and development roller 103) of the development blade 104, although having not been welded thereto, but having adhered fast enough to make it impossible to blow them away with the use of an air brush or the like. The toner particles were dammed by the agglomerations of toner particles adhering to the area b of the development blade 104. Therefore, the toner layer became thinner on the areas corresponding in position to the agglomerations, effecting therefore images suffering from the density anomaly in the form of vertical streaks.
Next, referring to FIG. 19, the effects of the change of the potential of the development roller 103 (xe2x88x92300 V) and development blade 104 (xe2x88x92400 V) will be concretely described.
As the toner particles having been given triboelectric charge by the supply roller 105 reach the area (adjacencies of area b) next to the upstream border of the contact area between the development roller 103 and development blade 104, the reversal toner particles (positively charged toner particles) in the toner layer are attracted to the area b of the surface of the development blade 104 due to the potential difference between the development roller 103 and development blade 104. The greater the potential difference between the development roller 103 and development blade 104, the greater the attraction between the reversal toner particles and the surface of the development blade 104. Therefore, in terms of the unwanted solid adhesion of the toner particles to the development blade 104, the area b is worst.
As the amount of the toner particles solidly adhering to the area b increases, the toner particles being moved in the direction of the area b by the rotation of the development roller 103 are dammed by the toner particles adhering to the area b of the development blade 104. As a result, the toner layer on the peripheral surface of the development roller 103 becomes thinner, across the areas corresponding in position to the agglomerations of the toner particles adhering to the area b of the development blade 104, effecting images which are low in density across the solid area. Further, since the amount by which the toner particles adhere to the area b of the development blade 104 are uneven in terms of its lengthwise direction, effecting the unwanted vertical streaks, in addition to the reduction in the density of the solid area.
This problem becomes particularly conspicuous when an external additive G2, which is positive in inherent polarity, in addition to the external additive G1, which is negative in inherent polarity (which hereinafter will be referred to as negative external additive) is mixed as auxiliary particulate additive into the toner. The external additive particles G2 positive in inherent polarity (which hereinafter will be referred to as positive external additive) are sometimes added to the mixture of the toner particles and the external additive negative in inherent polarity, for the purpose of stabilizing the electric charge of the toner, adjusting the fluidity of the toner, and the like. The external additive particles G2 positive in inherent polarity move in the same fashion as the aforementioned reversal toner particles. Therefore, they also adhere, as do the reversal toner particles, to the area b, or the portion of the development blade 104 in the adjacencies of the contact area between the development roller 103 and development blade 104, on the free edge side of the development blade 104 with respect to the contact area, contributing to the formation of the images which are low in density across the solid areas, and/or have unwanted vertical streaks.
Further, the surface potential level of the toner layer is affected by the condition of the ambience in which the image forming apparatus is operated; it increases as the humidity increases, and decreases as humidity decreases. Therefore, the voltage applied to the development blade 104 must be precisely controlled.
When AC bias is constantly applied to the development blade 104 as disclosed in Japanese Laid-open Patent Application 58-153972, a toner cloud is generated, by the AC bias, in the area a, that is, the area on the immediately downstream side of the contact area between the development roller 103 and development blade 104. The generation of this toner cloud results in useless scattering of the toner particles, contaminating the interior of the apparatus. Also, as AC bias is applied to the development blade 104, xe2x80x9cattack noisesxe2x80x9d are generated by the AC bias due to the contact between the development roller 103 and development blade 104.
In the case of a conventional image forming apparatus which used nonmagnetic single-component toner, toner sometimes leaked from the developing device, soiling the recording medium surface. In particular, in the case of a conventional apparatus in which the blade bias power source and development bias power source were rendered the same in potential, the signs of toner leakage were conspicuous. Moreover, the usage of nonmagnetic single-component toner, the particles of which are high in the degree of sphericity, more frequently caused toner leak due to the higher toner fluidity, although it resulted in the formation of images superior in dot reproducibility.
It is thought that this toner leak occurred for the following reason:
The toner particles are borne on, and remain adhered to, the development roller 103 due to the electric charge of the toner particles and the physical attraction between the toner particles and development roller 103. Therefore, when the toner particles are not uniform in the amount of electric charge, that is, when the toner layer contains toner particles with a smaller amount of electric charge and toner particles with a larger amount of electric charge, the toner particles with a smaller amount of electric charge are not as firmly held to the development roller 103 as the toner particles with a larger amount of electric charge, and therefore, they leak.
The detailed examination of the leaked toner particles with a smaller amount of electric charge revealed that the leaked toner particles are agglomerations of the reversal toner particles, that is, positively charged toner particles, and the normal toner particles, that is, negatively charged toner particles.
The primary object of the present invention is to provide a developing apparatus and an image forming apparatus, in which developer does not solidly adhere, nor weld, to the developer regulating member thereof.
Another object of the present invention is to provide a developing apparatus and an image forming apparatus, in which the amount by which developer is borne on the developer bearing member remains constant.
Another object of the present invention is to provide a developing apparatus and an image forming apparatus, in which developer does not leak during image formation.
Another object of the present invention is to provide a developing apparatus and an image forming apparatus, which are capable of reliably form, for a long period of time, images which do not suffer from the insufficient density and unwanted vertical streaks.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.